What are the difficulties the animals encounter building the windmill, and how do they overcome these?
The animals are faced with the challenge of trying to use human tools to work with their non-human bodies. For instance, they learn after repeated attempts that they can't break the quarry boulders with human tools like picks, so they devise a laborious process in which they drag the boulders up and out of the quarry, then push them back over the edge of the quarry to crash and shatter below. They then go and gather the pieces, which is not as hard for them as dragging the boulders. Nevertheless, the work is labor intensive, so they toil through 60-hour work weeks in order to accomplish the task. The conviction that they are working for their own greater collective good motivates them. Further, when they begin to flag, Boxer's strength and confidence rallies them. This episode reveals the ingenuity of the animals but also Napoleon's devious ways, such as saying work on the windmill is voluntary but that any animal who doesn't volunteer will be put on half rations.
Napolean tells the other animals that the project will require harder work and reduced rations for all of the animals, except, of course, the pigs. Life for the animals begins to get worse. They work harder and longer, 60 hours a week, including Sundays. Orwell writes in the beginning of chapter VI:
"All that year the animals worked like slaves. But they were happy in their work; they grudged no effort or sacrifice, well aware that everything that they did was for the benefit of themselves and those of their kind who would come after them, and not for a pack of idle, thieving human beings."
Because of the push to complete the windmill, the farm is neglected. Shortages necessitate trade with humans which causes another violation of the “unalterable” commandments of Animal Farm.